Scanning
probe microscopy (SPM)
is a family of mechanical probe microscopes
including scanning tunneling microscopy and atomic force microscopy
(AFM) that
measures surface morphology in real space with a resolution down to the
atomic level. Contact mode AFM is a mechanical probe
technique capable of
mapping real-space 3-demensional surface morphology and properties
such as friction force, adhesion
force and elasticity.

In the dynamic
force AFM mode, the
tip-sample interaction is controlled by maintaining a reduced amplitude
of a vibrating cantilever driven by a piezo element while the tip scans
the sample surface. Under this feedback mechanism, the phase lag
between the vibrating cantilever detected by the photodetector and the
driver signal, referred to as phase shift, is sensitive to both
mechanical and chemical properties of the sample. Mapping of the phase
shift (simultaneously with the topographic image), known as phase
imaging technique, has found applications in studies of soft materials.

Our research activities include
visualization of biological structures present on unfixed rat brain
sections and additives in polymer composites with phase shift
imaging. Shown above is
the topographic image and phase image (scan area is 5 micron square)
obtained on the surface of an
unfixed rat brain section, as well as amplitude- and phase-distance
curves. The contrast in phase image is a
reflection fact that
the energy dissipation of the vibrating cantilever/tip ensemble is
dependent on the mechanical
properties of the probed area.

The interactions between the probe and the sample surface
contain information in regard to viscoelasticity, surface
chemistry and other properties. Because this is a two-body
interaction, both the sample and probe contribute to the outcome of
their interaction. We are interested in controlling the
surface chemistry of AFM tips. Silicon probes are
inherently hydrophilic, making them prone to "sticky"
contaminants such as organic and inorganic oxides. A tip
modified
with hydrophobic coatings such as self-assembled monolayrs will
likely solve this type of problems.

SPM is used to measure surface features at nanometerscales because
silicon probes have a sharp tip (apex radius ~10 nm).
However, when the size of surface features are comparable to the tip
apex itself, the geometry of the tip apex will be convoluted to the
image. When the tip apex is larger than the surface features,
the image is no longer a reflection of the sample; rather, it is
dominated by the tip geometry. Understanding this nature of
AFM is important in properly interpreting images, especially
for nanometer-scale features.

2. Time-of-flight
secondary ion mass spectrometryBy measuring mass to charge ratios (m/z) of
secondary ion fragments generated from the bombardment of
a sample surface by a primary ion beam (e.g., Bi3+),
ToF-SIMS is a powerful tool in identifying molecules by detecting
molecular ion fragments and/or a number of characteristic
fragments. Its applications include verification and/or
identification of surface contaminants. For
example, a common
surface contaminant is silicone oil or polydimethylsiloxane, which has
been
found to result in a group of ion fragments serving as fingerprints
(for example, positive secondary ions at m/z 28, 73, 147, 207, 221 and
281, which are assigned to Si+,
SiC3H9+,
Si2C5H15O+,
Si3C5H15O3+,
Si3C7H21O2+
and Si4C7H21O4+,
respectively).
With
another ion gun used to sputter the surface, ToF-SIMS is capable of
depth profiling, for example, thin oxide layers of metals and layered
structures of electronic and optical films. It also
has
imaging capacity because a full spectrum is collected over each pixel
of the rastered area: an image of a selected ion fragment is generated
by plotting its intensity against the pixels.

This surface analytical technique can also be used to explore how
molecules are attached to a substrate by pondering (data mining) the
fragmentation of the molecules themselves and their association
with the substrate. For example,
as shown in the right, for octadecylphosphonic acid (OPA, or ODPA) SAMs
spin-coated on silicon and aluminum oxides, ToF-SIMS revealed
that ion fragmentation of the
OPA molecules (C18H39PO3, represented
by M) is distinctive: on silicon oxide,
there are
condensed dimers of [M2-H2O]-
and [M2-HO]+;
by contrast, there are no such ion fragments detected
on aluminum
oxide. OPA powders (can be called free OPA and the molecules
are
bonded by H-bonding between the headgroups and van der Waals
forces
between the alkyl chains) rendred same ion fragmentation as OPA SAMs
on silicon, suggesting that the molecules in the SAMs are bonded by
H-bonding with the substrate. The lack of these condensed
dimers,
plus other difference in ion fragmentation found
between OPA SAMs on
silicon and on aluminum oxides, prove that the OPA SAMs are covalently
bonded
to aluminum oxide. 3. Self-assembled
monolayers (SAMs)

Via use of appropriate non-polar solvents
having a
dielectric
constant of ~4, we developed a novel method
of
delivering OPA SAMs on a surface that is
hydrophilic.
In this method, as
shown in the schematic to the right, the solvent (trichloroethylene,
TCE) is used as an active
medium
to force the molecular headgroups to concentrate
on
the medium surface, leading
to a fast growth of SAMs, as well as formation of SAMs
that either have a covalent bond or H-bonding with an oxide,
depending on the nature of the interaction between the OPA molecular
headgroup
and the substrate. Shown below is an example of formation of
full-coverage OPA SAMs on silicon oxide by adding solution
during spin-coating.

Because of this solvent-assisted mechanism, the only
requirement for
SAM formation is physical contact between the OPA-loaded medium and a
hydrophilic substrate. This leads to an extremely fast growth
rate allowing spin-coating SAMs on a hydrophilic substrate. The utility
of
this method has been demonstrated by other researchers in modifying the
gate dielectric surface before depositing the active organic
semiconductor in organic thin-film transistors (OTFTs) for improving
device performance.

Our objective in this research area is surface engineering at the
molecular level with SAMs and utilizing such a well-controlled surface
to construct patterned surface chemistry for manipulation of chemical
and biological interactions.